3.3

• Plants need water for:

  • Photosynthesis – to create glucose​
  • Structure/support – vacuole presses cytoplasm against cell walls  turgidity​
  • Transport of dissolved nutrients​
  • Cooling​
• Waxy cuticle has to help prevent water loss
• The upper epidermis is thin and transparent to allow more light through to the chloroplasts
• Palisade cells are full of green chloroplasts, containing chlorophyll to trap sunlight
• Spongy mesophyll contains lots of air spaces to allow carbon dioxide to diffuse through the leaf and increase the surface area.
• Stomata are holes in the leaf to allow carbon dioxide to diffuse in and oxygen to diffuse out
• Guard cells open and close the holes to prevent water loss
• Size and exchange in plants

  • Plants continue to grow ​throughout their lives.​
  • Although some plants are small, ​many perennial plants (plants ​that live a long time and reproduce year after year) are large.​
  • This means plants need very effective transport systems to move substances both up and down from the top of the roots to the topmost leaves and stems.​
• SA:V ratio and exchange in plants

  • SA:V ratios are not simple in plants.​
  • Leaves are adapted to have a large SA:V ratio for gas exchange with the air.​
  • However, the size and complexity of multicellular plants means that when the stems, trunks, and roots are taken into account, they still have a relatively small SA:V ratio.​
  • This means they cannot rely on diffusion alone to supply their cells with everything they need.​
• Metabolic demands and exchange in plants

  • Non-green parts of plant (e.g. roots) can’t photosynthesis.​
  • They need glucose and oxygen (for respiration) transported to them.​
  • They need waste products of metabolism removed.​
  • Hormones made in one part of a plant need transporting to their target tissue.​
  • Mineral ions absorbed by the roots need to be transported to all cells to make the proteins required for enzymes and the structure of the cell.​
• Plants with specialised transport systems are vascular plants.​
• Plants exchange and transport:

  • Carbon dioxide – for photosynthesis​
  • Oxygen – for respiration​
  • Water – for photosynthesis, structure/support, transport of dissolved nutrients, cooling​
  • Organic nutrients (e.g. sugars, starch) – for respiration, storage​
  • Inorganic ions (e.g. nitrate, phosphate, potassium) – for healthy growth, making proteins, making chlorophyll, etc​
• Cotyledons = organs that act as food stores for a developing embryo plant, and form the first leaves when the seed germinates.​
• Dicots (dicotyledonous plants) = plants that make seeds containing two cotyledons.
• Herbaceous dicots = dicots with soft tissues and a relatively short lifecycle
• Transpiration​

  • Water and mineral ions​
  • Xylem vessels​
  • Up only​
  • Passive process
• Translocation​

  • Sugars & amino acids (assimilates)​
  • Phloem​
  • Bidirectional​
  • Active process​
• • Dicots have vascular tissue distributed throughout the plant.​
  • The xylem and phloem are found together in vascular bundles.​​
  • Xylem – dead tissue (transports water and minerals)​
  • Phloem – sieve tube elements and companion cells (transports assimilates such as sugars)​
• In roots and stem, the xylem tissue is found on the inside.​

  However, in leaves, xylem is found above phloem tissue.​
• Vascular bundle in leaves

  • The vascular bundles form the midrib and veins of a leaf.​
  • A dicot leaf has a branching network of veins that get smaller as they spread away from the midrib.​
  • Within each vein, the xylem is located on top of the phloem
  • In between the xylem and phloem is a layer of cambium.
• Vascular bundle in roots

  • The vascular bundle is in the centre of a young root.​
  • There is a central core of xylem, often in the shape of an X
  • The phloem in between the ​arms of the X
  • This arrangement provides ​strength to withstand the ​pulling forces to which roots ​are exposed.
  • Around the vascular bundle is a special sheath of cells called the endodermis.​
  • This has a key role in getting water into the xylem vessels. ​Just inside the endodermis is a layer of meristem cells called the pericycle.
• Vascular bundles also contain other types of tissue to give the bundle some strength and help to support the plant:​

  • Sclerenchyma​
  • Collenchyma​
  • Parenchyma​
• Sclerenchyma​

  • Support and structure (around vessels).​
  • Thickened with lignin (so cells are dead) and cellulose (more than in typical cells).​
  • They strengthen stems and leaf midribs.​
  • Fibres are one type of sclerenchyma cell.​
• Collenchyma​

  • Found by epidermis and involved in growth.​
  • They have thick cellulose walls, giving strength to vascular bundles and outer parts of stems.​
  • Flexible support providing wind resistance.​
• Parenchyma​

  • A soft packing tissue in plants which fills spaces between other tissues.​
  • In roots, parenchyma cells may store starch.​
  • In leaves, some have chloroplasts and can photosynthesise. ​
  • In aquatic plants, parenchyma has air spaces to keep the plant buoyant.
• • Xylem's main functions are support and water/mineral transport.​
  • The flow of materials in the xylem is up from the roots to the shoots and leaves.​
  • The xylem also help to structurally support the plant.​
• Xylem is made up of several types of cells, most of which are dead when they are functioning in the plant:​

  • Vessels​
  • Parenchyma​
  • Fibres​
• • The xylem vessels are made by several columns of cells fusing together end to end.​
  • Water and mineral ions are transported are transported through this tube.
• Adaptations of xylem vessels include:​

  • The contents of the cells decay, so the cells do not contain any cytoplasm or organelles (as these would slow down the flow of water). This leaves a long, hollow tube.​
  • The end plates break down, allowing unimpeded flow of water.​
  • The cell walls are thicker than usual, to withstand the pressure of the water flowing through.​
  • Cell walls are lignified in patterns (spiral, rings, or broken rings). ​
• Lignin kills the cells, but:​

  • Makes the cells waterproof​
  • Adds strength to withstand the pressure of the moving water​
  • Keeps the vessels open at all times​
  • The patterns of deposition allow some flexibility
• • Bordered pits = small non-lignified regions of the xylem vessel walls.​
  • They allow water and mineral ions to move laterally (sideways) between xylem vessels.
• Tannin = a bitter-tasting chemical that protects plant tissues from attack by herbivores, and infection from bacteria and fungi.
• • Transpiration = the loss of water from the stomata in the leaves as a result of evaporation.​
• • The main function of the phloem tissue is translocation.​
  • This is the transport of assimilates (organic compounds, particularly sucrose) from sources (e.g. leaves) to sinks (e.g. roots).​
  • These assimilates are dissolved in water to form sap.​
  • They are transported up and down the plant.​
• • Phloem tissue mostly consists of sieve tube elements and companion cells.​
  • Other cell types include parenchyma for storage, and strengthening fibres.​
  • Mature phloem tissue contains living cells, unlike xylem tissue.​
• • Sieve tube elements line up end to end.​
  • They are alive.​
  • Their function is to transport assimilates around the plant.​
• Adaptations of sieve tubes include:​

  • Elongated​
  • No nucleus​
  • Very little cytoplasm​
  • Tonoplast (vacuole membrane) and other organelles break down.​
• • Unlike in xylem vessels, the sieve tube's end walls do not disappear, but instead form sieve plates (perforated cross walls).​
  • The strands of cytoplasm can pass through the sieve plates.​
  • Sieve tube elements cannot keep themselves alive and need to be aided by companion cells (like a life support system).​
  • Companion cells and sieve tube elements are linked by plasmodesmata – gaps in cell walls which allow the cytoplasm to link.​
• Companion cells carry out the metabolic processes (e.g. respiration, excretion) needed to load assimilates into the sieve tubes.​
• Companion cell adaptations​:

  • Large nucleus​
  • Dense cytoplasm​
  • Many mitochondria to provide ATP for active transport​
  • Many infoldings in their cell surface membranes to give an increased surface area for active transport​